Control system for terminating the discharge of a flash lamp

ABSTRACT

An arrangement for controlling the flash duration in photographic cameras. A light-sensitive element within the apparatus responds to light originating from the source for flash bulbs and reflected by the object or scene to be photographed. The flash bulb is ignited by an ignition circuit which also generates the operating voltage for the light-sensitive device. Through the application of a discharge tube which has a considerably lower internal resistance, when ignited, than the flash bulb, the latter becomes extinguished when the .Iadd.quantity of .Iaddend.light reflected by the object or scene and impinging upon the light-sensitive device, is .[.of.]. a predetermined .[.magnitude.]. .Iadd.quantity..Iaddend.

BACKGROUND OF THE INVENTION

The present invention resides in an electronic flash apparatus forphotographic applications which includes a light meter fed from a DCoperating voltage supply. The electronic control circuitry containingthe light meter extinguishes the flash bulb used in conjunction with theflash apparatus as soon as the scene to be photographed has reflected apredetermined amount of light onto the light-sensitive element of thelight meter.

Heretofore, an electronic flash apparatus has been available for thepurpose of omitting the requirement to set the diaphragm of aphotographic camera prior to each exposure because of the differentdistances between the flash apparatus and the scene to be photographed.The flash apparatus serves under these conditions the purpose ofilluminating the scene to be photographed. With such electronic flashapparatus, the flash bulb becomes automatically extinguished when thescene to be photographed is sufficiently illuminated. In this manner,the object or scene being photographed is correctly illuminated in viewof the condition that the duration of the flash is controlled throughthe amount of light reflected by the scene upon a light sensitiveelement.

The light meter includes, for this purpose, an integrating arrangementwhich integrates the electrical current derived from the light-sensitiveelement and actuates in response thereto, a control arrangement, as soonas the integrated current attains a predetermined value. Theconventional means for integration is in the form of a capacitor whichintegrates the current derived from the light-sensitive element. Thecapacitor becomes charged, and when the voltage across the capacitorattains a predetermined value, the control arrangement becomes actuatedand the flash bulb becomes thereby extinguished. It is known in the artthat for the purpose of extinguishing the flash bulb, a discharge tubeis used in parallel with the flash bulb. The discharge tube has aconsiderably lower internal resistance than the flash bulb when it isignited. The discharge tube becomes thus ignited through the controlarrangement or circuitry when the capacitor has attained a predeterminedvoltage level. The lower internal resistance of the ignited dischargetube causes the discharge of the storage capacitor which provideselectrical energy for the flash bulb, and as a result the flash bulbbecomes extinguished.

An electronic arrangement provided with such a control circuit, however,has the disadvantage that the photographic process or operation can beinterfered with through the prevailing ambient light, as well as throughthe photographic flash resulting from another camera in the neighboringvicinity. The ambient light causes current to be generated through thelight-sensitive element before actuation of the flash apparatus, andthis current is applied to the capacitor which performs its integratingfunction. As a result, the capacitor possesses unpredictable initialvoltage prior to initiation of the flash. In view of this condition, thetime interval during which the predetermined voltage across thecapacitor is attained, is no longer well defined. Furthermore, a remoteflash from another apparatus in the vicinity, can actuate the controlarrangement or circuitry of the flash apparatus under consideration, atan undesired instant of time. The result of such action is that thestorage capacitor becomes thereby discharged.

In order to avoid interference through the ambient light, it has beenpreviously proposed to omit the integrating capacitor and to provide asingle and sole photoresistor of predetermined characteristics. Withsuch an arrangement, however, interference resulting from remote flashescan still not be avoided.

It is also known in the art, to connect the light meter into the circuitfirst at the beginning of the flash through a switch or shuttermechanism. With such design, disturbances or interferences can beavoided from either the ambient light or remote flashes. Such design,however, requires rather careful and meticalous mechanical constructionwhich is considerably complex and thereby costly to manufacture. At thesame time, such mechanism and design does not operate reliably.

In accordance with the present invention, on the other hand, anarrangement is provided in which the ambient light as well as remoteflashes are taken into account through a very simple and reliablemanner. The present invention provides that the operating DC voltage isrealized through the ignition of the flash bulb by an ignition currentfrom an ignition circuit for the flash bulb. The arrangement inaccordance with the present invention is based on electrical structurerather than mechanical parts, and thereby the control circuitry isprincipally switched on only during the flash. In this manner,interferences from remote flashes and ambient light cannot be incurred.

The flash bulb of an electronic flash apparatus becomes ignited throughthe application of an ignition capacitor operating in conjunction withan ignition transformer. The ignition capacitor is connected to theprimary winding of the ignition transformer, and when a synchronizing.[.shutter.]. .Iadd.trigger .Iaddend.cable becomes switchingly actuated,the ignition capacitor discharges and thereby produces a high voltageignition pulse in the secondary winding of the ignition transformer.This high voltage pulse is applied to the flash bulb.

In carrying out the concept of the present invention, the primarywinding of the ignition transformer can form a closed series circuitwith the ignition capacitor and with a charging capacitor, when ignitingthe flash bulb. The voltage across the charging capacitor results from.[.charging.]. .Iadd.discharging .Iaddend. of the ignition capacitor andserves as the DC operating voltage for the light meter.

When the electronic flash apparatus is switched on, one electrode of theignition capacitor is connected to a voltage divider associated with theflash bulb. The other electrode of the ignition capacitor leads toground potential, by way of the primary winding of the ignitiontransformer. The ignition capacitor becomes thus charged to acorresponding voltage.

When the synchronizing shutter cable is actuated so that it forms aclosed circuit through it, the charging capacitor becomes switched tothe ignition capacitor. One electrode of this charging capacitor isconnected to ground potential. With this arrangement the primary windingof the ignition transformer forms a closed series circuit with theignition capacitor and with the charging capacitor. The ignitioncapacitor, thereby, transfers a portion of its charge to the chargingcapacitor, by way of the primary winding of the ignition transformer.When, in accordance with the further provision of the present invention,the capacitance of the charging capacitor is large compared to that ofhte ignition capacitor, the largest proportion of the charge of theignition capacitor becomes transferred to the charging capacitor. As aresult, the equalizing current flows from the charged ignition capacitorto the uncharged capacitor by way of the primary winding of the ignitiontransformer. Through this current flow through the ignition transformer,the flash bulb becomes ignited.

The voltage resulting across the charging capacitor through rapidcharging of the latter to its maximum value, remains first constant nearthis maximum level, and forms the DC operating voltage for the lightmeter.

In a further embodiment of the present invention, the light meter cancontain a series circuit to which the DC operating voltage is appliedand which includes a light sensitive element in the form of aphotoresistor, an adjustable charging resistor, and an integratingcapacitor.

The adjustable charging resistor must be set in accordance with thesensitivity or speed of the film being used. This setting remains as theonly setting to be carried out by the photographer. The setting canremain for as long as the same film material is used. After the flashbulb becomes ignited, the current produced by the photoresistor becomesintegrated by the capacitor. This current through the photoresistor is afunction of the brightness or illuminating intensity and the distance ofthe flash bulb from the scene or object to be illuminated. Theintegrating capacitor attains a predetermined voltage after apredetermined time interval suitable for illuminating the film.

In accordance with a further embodiment of the present invention, thevoltage of the integrating capacitor can be applied, through aprotective resistor, to the cathode-control electrode path of acontrolled silicon diode or thyristor. This controlled silicon diode orsilicon rectifier ignites a discharge tube when controlled so that it isin the conducting state. This discharge tube lies in parallel with theflash bulb, in the conventional manner. The internal resistance of thedischarge tube, when ignited, has a considerably lower value than theinternal resistance of a flash bulb.

Through the large current due to the ignition of the discharge tube bythe thyristor, the storage capacitor which feeds the flash bulb becomesdischarged at an earlier instant of time than if it were to bedischarged only through the flash bulb itself. As a result, the flashbulb becomes extinguished at the correct instant of time determined bythe illumination required for the specific film being used in thecamera.

Another specific feature of the present invention resides in the mannerin which the thyristor and the discharge tube are interconnected.

SUMMARY OF THE INVENTION

An electronic flash arrangement for photographic cameras in which theflash bulb becomes extinguished after ignition when light reflected fromthe object to be photographed has attained a predetermined magnitude. Alight measuring circuit responds to light originating from the flashbulb and reflected by the object to be photographed. The flash bulb isignited through an ignition circuit which also generates a DC operatingvoltage for the light measuring device. Once the flash bulb becomesignited, light is reflected by the object and impinges upon the lightmeasuring device. When the reflected light impinging on the lightmeasuring device has thus attained a predetermined level, the flash bulbbecomes extinguished, and as a result the correct amount of illuminationis applied for the photographic operation.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

An electrical schematic diagram of the electronic flash arrangementshowing the operating components and their interconnections, inaccordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawing, a storage capacitor 1 of approximately 200 μfbecomes charged to approximately 500 volts during several seconds afterthe electronic flash apparatus is switched on through electricalelements, not shown. This storage capacitor 1 provides the electricalenergy for a flash bulb 2.

When the electronic flash circuit, of the present invention, is switchedon, one electrode or terminal of the capacitor 4 leads to the junctionof resistors 21 and 22, by way of resistor 23. Resistors 21 and 22 forma voltage divider connected across the 500 volt supply which is alsoapplied across the flash bulb 2. Thus, the voltage divider composed ofresistors 21 and 22 is connected in parallel with the terminals of theflash bulb 2. Whereas one terminal of the resistor 21 is connected tothe +500 volt potential line, the resistor 22 terminates at groundpotential. The capacitor 4 functions in the circuit as an ignitioncapacitor. The primary winding of an ignition transformer 3 has oneterminal connected to the opposite terminal of the ignition capacitor 4,so that in this other or opposite terminal of the ignition transformer 4leads to ground potential through the primary winding of the ignitiontransformer 3. The ignition transformer serves further to ignite theflash bulb, and as a result of connecting the primary winding of thetransformer 3 to the capacitor 4, in this manner, the ignition capacitorbecomes charged to a voltage established by the voltage dividerconsisting of resistors 21 and 22.

The switch 20 shown in the drawing represents the shutter synchronizingand actuating cable of the camera. This cable is between the flashapparatus and the photographic camera. When this synchronizing cable 20is actuated, a short circuit is established across the terminals,denoted in the drawing as associated with the switch 20, and a seriescircuit is formed through the primary winding of the ignitiontransformer 3, the ignition capacitor 4, and a charging capacitor 5. Asa result of this closed series circuit, the ignition capacitor 4transfers charge to the charging capacitor 5, so that in the final stateboth capacitors have the identical voltage applied across them. Thecurrent flowing from the ignition capacitor 4 to the charging capacitor5 for the purpose of equalizing their charge voltages, flows alsothrough the primary winding of the ignition transformer 3. As a resultof this current flow through the primary winding, a high voltage pulseis generated in the secondary winding of the transformer, and the flashbulb 2 becomes ignited.

Assuming for example, that the ignition capacitor 4 has a capacitance of0.2 μf, then the voltage prevailing across the capacitor 5, when thecircuit is switched on, is 1/11 of 250 volts, or approximately 23 volts.This voltage which remains first constant forms the operating DC voltagefor a light meter which contains the series circuit comprised of aphotoresistor 6, a variable or adjustable resistor 7, and an integratingcapacitor 8.

The photoresistor is a light-sensitive element and varies its resistanceas a function of the illumination intensity. For larger illuminatingintensities the resistance decreases and permits a larger current toflow from the charging capacitor 5 to the capacitor 8. The latterintegrates the individual current elements to the extent that thevoltage on the capacitor 8 thereby increases. When illumination oflarger magnitudes is applied to the photoresistor 6, the capacitor 8attains more rapidly a predetermined voltage level .Iadd.herein referredto as terminating signal.Iaddend., than it does when the illuminationintensity is of lower magnitudes. The adjustable resistor 7 must be set,as already indicated, in accordance with the sensitivity of the filmmaterial being used.

A control silicon diode 11 in the form of a thyristor receives its anodepotential by way of a large resistor 12 which may be of the order ofapproximately 10 megohms. The resistor 12 is connected to the tap orjunction 19 of a voltage divider comprised of resistors 13 and 14. Thisvoltage divider is connected across the voltage supply of 500 volts forthe flash bulb 2. The control or starting electrode of the thyristor isconnected with the integrating capacitor 8, by way of a protectiveresistor 9. The cathode of this thyristor, on the other hand, isconnected to ground potential. The anode-cathode path of the thyristoris not conducting when the voltage between the control electrode and thecathode is below a predetermined value. As long as the thyristor isnonconducting, the ignition capacitor 15 for a discharge tube 17, alsoremains charged. The ignition capacitor 15 is connected to the junction18 leading to the anode of the thyristor, and leading also to the tap 19of the voltage divider of resistors 13 and 14, by way of the largeresistor 12. The other terminal of the capacitor 15 leads to groundpotential by way of the primary winding of an ignition transformer 16.

When ignition of the flash bulb 2 is initiated through the shortcircuiting cable switch 20, the voltage across the charging capacitor 5becomes available as a DC operating voltage. The integrating capacitor 8thus acquires a voltage sufficient to fire the thyristor 11, .Iadd.thisvoltage being herein referred to as a terminating signal.Iaddend., as aresult of the current flow from the charging capacitor 5 and through thephotoresistor 6. When the thyristor is thus fired and the anode-cathodepath of the thyristor is conducting, the ignition capacitor 15 for thedischarge tube 17, discharges through the thyristor. As a result of thisdischarge, a current pulse is realized in the primary winding of theignition transformer 16. The discharge tube 17 becomes ignited throughthe secondary winding of this transformer 16. This discharge tube 17,when ignited has a considerably lower resistance than the flash bulb 2,and therefore the latter is extinguished when the discharge tube becomesignited. As a result of such action, the object or scene to bephotographed is no longer illuminated. The discharge tube 17 is mountedwithin the interior of the electronic flash apparatus, and is coveredagainst the exterior environment.

After the flash bulb 2 is extinguished through the discharge tube 17, asmall residual voltage prevails across the storage capacitor 1 and alsoat the tap or junction 19 of the voltage divider comprised of resistors13 and 14. In order that the voltage at the anode of the thyristor 11,and thereby the junction 18, acquires a small value for the purpose ofagain turning off the thyristor, a large ohmic value is selected for theresistor 12. Thus, this resistor is selected so that it has a value ofthe order of 10 megohms. The integrating capacitor 8.[.charges.]..Iadd.discharges.Iaddend., after control of the thyristor11 has been exercised, through the protective resistor 9 and thecathode-control electrode path towards ground potential.

Through closing of a switch 10, the electronic control arrangement forautomotic.]. .Iadd.automatic .Iaddend.extinguishing of the flash bulb 2,may be made inoperative.

The component values indicated in the circuit diagram of the drawing,have been designated only for the purpose of illustrating the basis forthe selection of the components. It is quite possible to provide, incases, different values for these components.

It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types ofconstructions differing from the types described above.

While the invention has been illustrated and described as embodied inelectronic flash apparatus for cameras, it is not intended to be limitedto the details shown, since various modifications and structural changesmay be made without departing in any way from the spirit of the presentinvention.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims:

I claim: .[.1. An electronic flash arrangement for photographic camerascomprising, in combination, flash bulb means; ignition means connectedto said flash bulb means for igniting said flash bulb means; lightmeasuring means for responding to light from said flash bulb means andreflected by the object to be photographed; said ignition meansconnected to the light measuring means in a way as to deliver a D.C.operating voltage for the light measuring means, so that said D.C.operating voltage prevails only after the ignition of the flash bulbmeans, said flash bulb means becoming extinguished after ignition whenthe light reflected by said object and impinging upon said lightmeasuring means is of a predetermined magnitude..].
 2. The electronicflash arrangement .[.for photographic cameras.]. as defined in claim.[.1.]. .Iadd.12 .Iaddend.wherein said ignition means comprises anignition transformer connected to said flash .[.bulb.]. means; .[.and.]..Iadd.an .Iaddend.ignition capacitor connected to said ignitiontransformer means; and a charging capacitor connected in series withsaid ignition capacitor and said ignition transformer means when saidflash bulb means is ignited, the voltage across the charging capacitorresulting from .[.charging.]. .Iadd.discharge of .Iaddend.said ignitioncapacitor and being said .Iadd.D.C. .Iaddend.operating voltage foroperating said light measuring means.
 3. The electronic flasharrangement .[.for photographic cameras.]. as defined in claim .[.2.]..Iadd.14 .Iaddend.wherein the capacitance of said charging capacitor islarge compared to the capacitance of said ignition capacitor.
 4. Theelectronic flash arrangement .[.for photographic cameras.]. as definedin claim 3 wherein the capacitance of said charging capacitor is tentimes as large as the capacitance of said ignition capacitor.
 5. Theelectronic flash arrangement .[.for photographic cameras.]. as definedin claim .[.1.]. .Iadd.12 .Iaddend.wherein said light measuring meansincludes the series connection of a light sensitive element, anadjustable resistor, and an integrating capacitor.
 6. The electronicflash arrangement .[.for photographic cameras.]. as defined in claim 5wherein said light sensitive element is a photoresistor.
 7. Theelectronic flash arrangement .[.for photographic cameras.]. as definedin claim 5 including controlled silicon rectifying means withcathode-control electrode path connected to said integrating capacitormeans, discharge tube means .Iadd.connected in .Iaddend.parallel withsaid flash bulb means and ignited by said controlled rectifier means,the internal resistance of said discharge tube means being substantiallysmaller when ignited than the resistance of said flash bulb means. 8.The electronic flash arrangement .[.for photographic cameras.]. asdefined in claim 7 including protective resistor means between saidintegrating capacitor means and said .[.controlled.]..Iadd.control.Iaddend.electrode-cathode path of said controlled rectifier means. 9.The electronic flash arrangement .[.for photographic cameras.]. asdefined in claim 7 including an ignition transformer connected to saiddischarge tube means for igniting the same; ignition capacitor meansconnected between said ignition transformer means and said controlledrectifier means; and voltage divider means connected to said ignitioncapacitor means and said controlled rectifier means.
 10. The electronicflash arrangement .[.for photographic cameras.]. as defined in claim 9including resistor means connected between said voltage divider meansand the junction of said ignition capacitor means and said controlledrectifier means.
 11. The electronic flash arrangement .[.forphotographic cameras.]. as defined in claim 10 wherein said resistormeans has a resistance value .[.substantially of.]. .Iadd.in.Iaddend.the order of megohms. .Iadd.
 12. An electronic flasharrangement for a photographic process, comprising in combination, flashgenerating means for generating a flash; ignition means connected tosaid flash generating means for igniting said flash generating meansupon external activation, thereby initiating said flash, said externalactivation creating a current flow in said ignition means, said ignitionmeans further comprising means for furnishing a D.C. voltage only afterthe start of said current flow; light measuring means operative onlyafter application of an operating voltage thereto, for responding tolight during an exposure and furnishing a terminating signal when saidlight has reached a predetermined light quantity; means connecting saidignition means to said light measuring means in such a manner that saidD.C. voltage constitutes said operating voltage; and means connected tosaid light measuring means for terminating said flash upon receipt ofsaid terminating signal. .Iaddend. .Iadd.
 13. An electronic flasharrangement for a photographic process, comprising in combination, flashgenerating means for generating a flash; ignition means connected tosaid flash generating means for igniting said flash generating meansupon external activation, thereby initiating said flash, said externalactivation creating a current flow in said ignition means, said ignitionmeans further comprising means for creating a D.C. voltage only afterthe start of said current flow; light measuring means operative onlyafter application of an operating voltage thereto, for responding tolight during an exposure and furnishing a terminating signal when saidlight has reached a predetermined light quantity; means connecting saidignition means to said light measuring means in such a manner that saidD.C. voltage constitutes said operating voltage; and means connected tosaid light measuring means for terminating said flash upon receipt ofsaid terminating signal. .Iaddend. .Iadd.
 14. An electronic flasharrangement for photographic process, comprising, in combination, flashgenerating means for generating a flash; ignition means connected tosaid flash generating means for igniting said flash generating meansupon external activation, thereby initiating said flash, said ignitionmeans comprising an ignition transformer connected to said flashgenerating means, an ignition capacitor connected to said ignitiontransformer and a charging capacitor connected to said ignitioncapacitor in response to said external activation in such a manner thata D.C. voltage is created across said charging capacitor resulting fromdischarge of said ignition capacitor; light measuring means operativeonly after application of an operating voltage thereto, for respondingto light during an exposure and furnishing a terminating signal whensaid light has reached a predetermined light quantity; means forconnecting said ignition means to said light measuring means in such amanner that said D.C. voltage across said charging capacitor constitutessaid operating voltage; and means connected to said light measuringmeans and said flash generating means for terminating said flash uponreceipt of said terminating signal..Iaddend.